In a utility grid, it is ideal that a balance between supply and demand is maintained, but the balance is sometimes deviated. The shifting of balance appears as a frequency variation. When a lack of power supply ability is caused because of a generator isolation and a grid failure, the grid disturbance (the frequency variation, and the like) occurs, and accordingly, without the control ability, the disturbance leads to corruption of the entire grid, thereby causing a risk of a massive power failure. Immediately after the corruption of the balance, a period of the frequency variation is short, and accordingly hydroelectric power stations which have high responsibility especially to an output demand value, for example, can contribute the control thereof. After the variation has been in a quasi-equilibrium state at a slightly-higher frequency after the variation calmed down, the means to stop the wind turbine has been taken for an object of suppressing the output.
As a reference example related to a control of a wind turbine generator in the frequency variation of utility grid, Patent Literature 1 is cited. In Patent Literature 1, a technique intended to suppress the variation that is given to the frequency of grid by a wind turbine generation is described. To achieve this object, a PID control and a control having a hysteresis each based on a deviation between a measured frequency of grid and a normal frequency are carried out.